With the miniaturization of semiconductor integrated circuits, it has become difficult to embed an insulating film with a CVD method. In order to ensure the embedding performance of an insulating film, a method of embedding polysilazane using an application method has been under consideration. Polysilazne refers to a polymer having SiH2—NH— as the basic structure, and can be transformed into a silicon oxide film when it is annealed in a water vapor atmosphere. Concerning the temperature of water vapor, a higher temperature gives a characteristic which is close to that of a silicon dioxide, and at the same time causes the side walls of Si to be easily oxidized, thereby bringing about the transformation of silicon. When the temperature of water vapor is low, the oxidation of the side walls of Si is suppressed, but the etching rate by an etchant increases. For this reason, it is difficult to obtain the desired shape.